1. Field of the Invention
This invention relates to vertical cavity surface emitting lasers (VCSELs). More specifically, it relates to distributed Bragg reflector (DBR) mirrors for VCSELs.
2. Discussion of the Related Art
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. While there are many VCSEL variations, a common characteristic is that VCSELs emit light perpendicular to a semiconductor wafer's surface. Advantageously, VCSELs can be formed from a wide range of material systems to produce specific characteristics.
VCSELs include semiconductor active regions, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Because of their complicated structure, and because of their specific material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
FIG. 1 illustrates a typical long-wavelength VCSEL 10. As shown, an n-doped InP substrate 12 has an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the InP substrate 12, and an n-type graded-index InP lower spacer 18 is disposed over the lower mirror stack 16. An InGaAsP or AlInGaAs active region 20, usually having a number of quantum wells, is formed over the InP lower spacer 18. Over the active region 20 is an insulating region 40 that provides current confinement. The insulating region 40 is usually formed either by implanting protons or by forming an oxide layer. In any event, the insulating region 40 defines a conductive annular central opening 42 that forms an electrically conductive path through the insulating region 40. Over the insulating region is a tunnel junction 28. Over the tunnel junction 28 is an n-type graded-index InP top spacer 22 and an n-type InP top mirror stack 24 (another DBR), which is disposed over the InP top spacer 22. Over the top mirror stack 24 is an n-type conduction layer 9, an n-type cap layer 8, and an n-type electrical contact 26.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As the optical cavity is resonant at specific wavelengths, the mirror separation is controlled to resonate at a predetermined wavelength (or at a multiple thereof).
In operation, an external bias causes an electrical current 21 to flow from the electrical contact 26 toward the electrical contact 14. The tunnel junction over the insulating region 40 converts incoming electrons into holes. The converted holes are injected into the insulating region 40 and the conductive central opening 42, both of which confine the current 21 such that the current flows through the conductive central opening 42 and into the active region 20. Some of the injected holes are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are very good reflectors, some of the photons leak out as light 23 that travels along an optical path. Still referring to FIG. 1, the light 23 passes through the conduction layer 9, the cap layer 8, an aperture 30 in electrical contact 26, and out of the surface of the vertical cavity surface emitting laser 10.
It should be understood that FIG. 1 illustrates a typical long-wavelength VCSEL, and that numerous variations are possible. For example, the dopings can be changed (say, by providing a p-type substrate), different material systems can be used, operational details can be tuned for maximum performance, and additional structures and features can be added.
While generally successful, the conventional VCSELs have problems with DBRs. Thus, it is beneficial to consider DBRs in more detail. A DBR in VCSELs is formed by depositing 30 to 50 alternating layers of different transparent materials. Each layer is one quarter of a wavelength thick and the index of refraction is different for the two materials. In general, there are three main requirements for DBR materials. First, the two materials stacked must have significantly different indices of refraction (high refractive index contrast) to achieve high reflectivity to reduce optical losses. Second, the materials must be compatible with the substrate used to grow the active region. Third, the materials should be thermally conductive as well to dissipate the heat built-up during the operation of VCSELs. One problem is the poor thermal impedance of DBR materials that degrades performance of the VCSELs. Long-wavelength VCSELs on an InP substrate and red-color VCSELs on a GaAs substrate especially suffer from this poor thermal impedance of DBR materials, since these VCSELs are currently employing a DBR material system that has a poor thermal conductivity.